There's no such thing as a "readonly" checkbox. They can be disabled.
CSS overrides, that you load after the JQM CSS. As you should do to alter the appearance of ANY CSS framework.
You will have to work with your desktop browser's CSS inspection/debug tools to figure it out.
Keep in mind JQM hides the checkbox HTML, and uses an ::after rule on the label to create the styled checkbox.
I fiddled with it a bit, using the example at:
but I have no incentive to spend time on it, since I haven't used JQM in my own work in years. (Nor have any other regular contributor here.)
The HTML for the disabled checkbox there:
<div class="ui-checkbox ui-state-disabled">
<label for="checkbox-t-3a" class="ui-btn ui-corner-all ui-btn-a ui-btn-icon-left ui-checkbox-off">One</label>
<input disabled="" type="checkbox" name="checkbox-t-3a" id="checkbox-t-3a" data-theme="a"
">
</div>
You'll use the ui-state-disabled class as part of your selector. I can't guess if you want to change the style of ALL or just one or some disabled checkboxes.
The JQM CSS is very complex. Good luck!
If this is a new project, I advise picking a different CSS/JS framework. JQM is a dead project, and will probably never be updated.